1. Field of the Invention
This invention relates to improvements in a focus detecting capability for an object of regular pattern in a divided pupil image deviation detection type focus detecting device.
2. Related Background Art
As a type of the focus detecting device of a camera, there is known a method whereby the exit pupil of a photo-taking lens is divided into two by an optical system for focus detection, two object images formed by light fluxes passed through the pupil areas are received by a photoelectric conversion element array (for example, a CCD sensor array), the focus state of the photo-taking lens is detected from the output thereof and the photo-taking lens is driven on the basis of the result of the detection.
In FIG. 1 of the accompanying drawings, a field lens FLD is disposed with its optic axis common to the optic axis of a photo-taking lens LNS whose focus is to be detected. Two secondary imaging lenses FCLA and FCLB are disposed rearwardly of the field lens at positions symmetrical with respect to the optic axis. Sensor arrays SAA and SAB are further disposed rearwardly of the secondary imaging lenses. Diaphragms DIA and DIB are provided near the secondary imaging lenses FCLA and FCLB, respectively. The field lens FLD substantially images the exit pupil of the photo-taking lens LNS on the pupil planes of the two secondary imaging lenses FCLA and FCLB. As a result, light fluxes entering the secondary imaging lenses FCLA and FCLB, respectively, emerge from regions of equal areas on the exit pupil plane of the photo-taking lens LNS which correspond to the respective secondary imaging lenses FCLA and FCLB and do not overlap each other. When an aerial image formed near the field lens FLD is re-imaged on the surfaces of the sensor arrays SAA and SAB by the secondary imaging lenses FCLA and FCLB, two images on the sensor arrays SAA and SAB change their positions on the basis of the displacement of the position of the aerial image in the direction of the optic axis. Accordingly, if the amount of displacement (deviation) of the relative position of the two images on the sensor arrays is detected, the focus state of the photo-taking lens LNS can be known.
The focus state detecting method as described above may sometimes not operate well depending on the conditions of an object the most typical case is being an object of low luminance. The photoelectric charge accumulation time of a photoelectric sensor is limited in practical use, and unless there is a quantity of light high enough to produce sufficient photo-electric charges during that time, a signal cannot be formed. In such a case, even if the accumulation time is extended, the camera will become very difficult to use or a dark current will increase and the actually effective S/N will not be improved. So, it is often practised to carry an auxiliary light for focus detection on the camera and supplement the deficiency of the quantity of light during low luminance. The upper optical system of FIG. 1 is an auxiliary light projection system, and the emitted light from a light source LED illuminates a pattern chart CHT through a condenser lens CON, and the pattern of the pattern chart CHT is projected onto the surface of an object by a light projection lens LEL. If as in FIG. 1, the photo-taking lens and the light projection lens are discrete from each other, there will occur parallax, but if light is projected from the photo-taking lens, ghost image is liable to occur and therefore, usually, a light projection system is discretely provided outside the photo-taking system.
It is known that auxiliary light projection is effective for not only objects of low luminance but also objects of low contrast. During low contrast, there is no brightness-darkness pattern which is the basis of calculation, and focus detection cannot be accomplished. In such a case, a pattern can be projected onto the surface of an object to thereby forcibly give the object a brightness-darkness pattern and accomplish focus detection calculation on the basis thereof.
FIG. 2 of the accompanying drawings shows an example of the photoelectric conversion outputs of two images formed on the sensor arrays SAA and SAB by the construction of FIG. 1. The output of the sensor array SAA is A(i), and the output of the sensor array SAB is B(i). The number N of the picture elements of a sensor needs to be a minimum of five, and desirably should be several tens or more. A signal processing method for detecting the amount of image deviation PR from the image signals A(i) and B(i) is disclosed by the applicant assignee in Japanese Laid-Open Patent Application No. 58-142306, Japanese Laid-Open Patent Application No. 59-107313, Japanese Laid-Open Patent Application No. 60-101513 or Japanese Patent Application No. 61-160824.
The amount of image deviation is found by the method disclosed in these patent applications and on the basis thereof, the focus adjustment of the photo-taking lens is effected, whereby the photo-taking lens can be brought into the in-focus state.
In the method disclosed in the above-mentioned patent applications, for example, for two image signals A(i) and B(i) (i=1, 2, . . . N), ##EQU1## is calculated with respect to an integer value m(the amount of shift). max {A, B} represents the greater one of two real numbers A and B. The range of i in which the sum is taken is determined from the condition that suffixes i, i+k-m, i+k and i-m must be in the closed section [i, n ]. k is an integer constant and usually, k=1. Also, the range of m is concerned with the degree of the amount of image deviation to be detected and is not unconditionally determined, but usually the correlation amount defined by the equation (1) in which m is varied within -N/2.ltorsim.m.ltorsim.N/2is an example, and the following principle also holds quite true of the other known correlation amounts than this.
As the correlation amount formula, for example, the following is adopted besides the equation (1): ##EQU2##
The result of the above-mentioned equation (1) having been calculated with respect to each m is as shown in FIG. 3 of the accompanying drawings, wherein the location of m at which V(m) is inverted in its sign is the amount of image deviation expressed in the unit of picture element pitch. This value usually does not assume an integer. Assuming that there is an inversion image deviation M.sub.O including a fraction can be calculated by EQU M.sub.O =m.sub.O .vertline.V(m.sub.O)/{V(m.sub.O +1)-V(m.sub.O)}.vertline.(2)
Means for dividing the exit pupil of the photo-taking lens, as disclosed in U.S. Pat. No. 4,185,191, besides the above-described example of the prior art, may be a number of units each comprising a minute lens disposed in front of a pair of photoelectric sensors and arranged on a straight line, and is not specifically restricted.
A focus detecting device based on the detection of the amount of image deviation as described above has the characteristic that generally it malfunctions for an object pattern of regularity. This drawback comes directly from the principle of detecting the deviation between two images.
Assuming, for example, that as shown in FIG. 4 of the accompanying drawings, the object image is a repetitive pattern of a period pf pitch P on the surface of a photoelectric sensor, if an attempt is made to make the positions of two images A(i) and B(i) coincident with each other, even if the image A(i) is shifted in the direction of arrow .alpha. or the image B(i) is shifted in the direction of arrow .beta., the two images can be made coincident with each other, and the amount of image deviation cannot be primarily defined. Further, the shift point at which the two images coincide with each other exists at each one pitch of regularity besides the above-mentioned .alpha. and .beta..
Due to the above-described circumstances, as regards an object having a regular pattern, the amount of image deviation cannot be detected by the prior-art method and accordingly, the defocus amount of the photo-taking lens cannot be calculated and the in-focus state of the photo-taking lens cannot be determined. Contrary to expectation, there are many regular patterns in artificial structures, such as window lattices, railings, blinds, check or striped cloth and lined-up bookshelves, and often these cannot be neglected as the objects of a camera.
The focus detecting method based on the detection for the amount of image deviation as described above has the characteristic that it is not only weak to said regular patterns, but also the error is generally great for the patterns of objects in which the fluctuation of brightness and darkness is gentle. For example, objects having illumination distributions as shown in FIGS. 12A-12C of the accompanying drawings are objects of low brightness/darkness deviation rate. Among these, the pattern of FIG. 12C is that of a so-called low contrast object and originally has a small fluctuation of brightness and darkness due to the deficiency of the absolute value contrast of the object. Here, the contrast is usually the absolute amount of a change in brightness and darkness defined by EQU C=max{A(i)}-min{A(i)}
or ##EQU3## from the image signal A(i) output from the photoelectric conversion element array. For a low contrast object as shown in FIG. 12C, there is known a method of detecting the state thereof and taking some countermeasure, as previously described. However, there are many cases where as shown in FIGS. 12A and 12B, the contrast defined by the equation representing said C is high, but the change in brightness and darkness is gentle and accurate focus detection cannot be accomplished. When the contrast is calculated with respect to these objects, there is obtained a value which does not differ greatly from the edge pattern of FIG. 12D, but actually, in a situation wherein the focus detection error is great and particularly, electrical noise and optical noise such as ghosts are added, accuracy is remarkably bad for an object having a gentle change is brightness and darkness. This is a problem common to the algorithms in which the zero cross point is found as in FIG. 3. In the pattern as shown in FIG. 12D wherein the difference between brightness and darkness is localized, the steep difference between the brightness and darkness thereof is grasped and therefore, the influence of noise is relatively small, while in a pattern wherein the difference between brightness and darkness changes gently, the correlation signal is liable to be influenced by electrical and optical noises. In fact, if the pattern of FIG. 12A and the pattern of FIG. 12D are compared with each other under the same condition of the contrast of the equation C=max{A(i)}-min{A(i)} and under the electrical noise condition of the same level, the focus detection accuracy differs twice or more therebetween, and if an optical ghost is further added, the difference increases further.
Furthermore, the focus detecting method based on the detection of the amount of image deviation as described above is bases on the premise that the pattern shapes of the two optical images A(i) and B(i) agree with each other and only their positions shift relative to each other and therefore, even the above-described calculation algorithm of the amount of image deviation is based on the fact that the pattern shapes of the two images are equal to each other, and if the shapes of the optical images differ from each other, it will immediately provide a factor of an error and the operating performance of the system will be remarkably deteriorated. Therefore, it is important in manufacture to maintain the shape similarity between the two optical images, and designing and manufacturing have been performed with close attention paid, for example, to the identity of the openings of the diaphragms DIA and DIB, the imaging balance of the secondary imaging lenses FCLA and FCLB, the sensitivity irregularity of the photo-electric conversion element arrays SAA and SAB, etc.
However, whatever consideration may be given to the focus detecting optical system, where there are two or more objects of different distances in the focus detection field seen by the photoelectric conversion element arrays SAA and SAB, or where due to a backlit situation of high intensity, a ghost occurring in the photo-taking lens LNS mingles with the focus detecting optical system, the shape similarity between the two optical images is not maintained. Where there are two or more objects of different distances in the focus detection field (the so-called far-and-near concurrence condition), the object field seen from each of the divided pupil areas has a parallax and therefore, under whatever condition the defocus state of the photo-taking lens LNS may be, the two optical images become such as shown in FIG. 19C of the accompanying drawings, and their shapes do not agree with each other. This is theoretical. Usually, in such case, the photo-taking lens is focused to one of the objects lying at different distances, or is focused to an intermediate distance between those objects, with a result that the lens is defocused to any object. In the former case, to whcih object the photo-taking lens should be focused cannot be foreseen and therefore, it is not always the main object intended by the photographer, and in the latter case, there is no object to which the lens should be focused and therefore, in any case, it is a problem.
A counter-light ghost occurs in the case of outdoor photographing with the sun placed in front of the camera, as well as in the case of an object such as a figure at the window or an object adjacent to artificial illumination. The distribution of the quantity of light by the ghost scarcely occurs in the same manner for two images and the level difference therebetween is great and therefore, the similarity between two images is remarkably broken as shown in FIGS. 19A and 19B of the accompanying drawings. Heretofore, it has usually been the case that under such conditions, the reliability of focus detection is reduced to result in malfunctioning or deterioration of the focusing accuracy. According to the most advanced method, such a situation can be recognized, but the manner of coping with such a situation is to stop the automatic focus adjusting function of the system and display that effect, or to change the calculation algorithm in accordance with the photoelectric conversion signal of a bad condition. The latter is to limit the actually effective focus detection field used in calculation by software, for example, in the case of far-and-near concurrence, and in a counter-light ghost, it is known to eliminate a ghost component including much of a relatively low spatial frequency component from the object pattern by digital filtering means.
The above-described method obtains a predetermined effect, but since the original image signal originates from inferior optical images having no coincidence in shape, it is often the case that the result of calculation is not sufficient in respect of reliability. Therefore, in a focus detecting device of the pupil division image deviation detection type, it has heretofore been an important task to maintain a highly reliable focus detecting operation even under such a bad condition.